The Roman Catholic Church in England and Wales is intensifying its campaign against the government's plan to legalise same-sex marriage.
In a letter being read in 2,500 parish churches, the Church's two most senior archbishops say the change would reduce the significance of marriage.
The letter says Roman Catholics have a duty to make sure it does not happen.
The government wants to introduce gay marriage by 2015, but says churches would not have to perform weddings.
Last week Cardinal Keith O'Brien, the leader of the Roman Catholic Church in Scotland, said the "grotesque" plans would "shame the United Kingdom in the eyes of the world" if implemented.
And on Friday, in a speech to visiting US bishops, Pope Benedict XVI warned of "powerful political and cultural currents seeking to alter the legal definition of marriage", in the wake of the US states of Washington and Maryland legalising same-sex marriage.
Civil partnerships were introduced in 2005 to give same-sex couples similar legal rights to married couples, but the law does not allow such unions to be referred to as marriages.
The letter by Archbishop Vincent Nichols, the leader of the Roman Catholic Church in England and Wales, and Archbishop Peter Smith - the Archbishop of Southwark - tells Catholics that changing the nature of marriage would be a "profoundly radical step" that would reduce its effectiveness and significance.
In one passage the archbishops write: "There are many reasons why people get married. For most couples, there is an instinctive understanding that the stability of a marriage provides the best context for the flourishing of their relationship and for bringing up their children.
"Society recognises marriage as an important institution for these same reasons: to enhance stability in society and to respect and support parents in the crucial task of having children and bringing them up as well as possible."
They also add that changing the law would "gradually and inevitably transform society's understanding of the purpose of marriage.
"There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children."
The letter ends by telling Catholics they have a "duty to do all we can to ensure that the true meaning of marriage is not lost for future generations".
BBC religious affairs correspondent Robert Pigott says Archbishop Nichols - the Archbishop of Westminster - has a record of mobilising the faithful.
To many Christians, while a civil partnership confers all the legal rights of marriage, a church wedding is a mystical event, the making of promises before God in a sacred setting, endowing the relationship with a special "blessed" quality, our correspondent says.
He adds that the letter is couched in "measured language" but it is intended to rally Catholics against the changes.
The leader of the Church of England, Archbishop of Canterbury Dr Rowan Williams, has said the law should not be used as a tool to bring about such social changes such as gay marriage, and may turn out to be ahead of majority opinion.
And the Archbishop of York, Dr John Sentamu, opposes gay marriage but supports civil partnerships, which he says have addressed the injustices faced by same-sex couples.
"There's a difference - and people don't these days want to talk about difference - there's a difference between a civil partnership and marriage, and that difference doesn't mean one is better than another, but they're different."
Lib Dem support
Ben Summerskill, from the lesbian and gay charity Stonewall, said most people who heard the letter would ignore its contents.
"It's a shame Catholic church leaders are so deeply opposed to a 21st-century balance of rights that they're not reading out letters about serious issues such as the Aids crisis in Africa or the 2.5 million children who live in poverty in this country.
"We're sure most churchgoers will be as opposed to their leaders on this issue as they are on birth control," he said.
Mark Dowd, from the group Quest, which represents lesbian and gay Catholics, said the archbishops were out of touch as other countries had begun to make changes.
"Probably the Archbishop resembles King Canute standing on the shores with the waves coming in. It's really a question of the tide of history turning and there's very little that can be done about it."
Speaking at the Liberal Democrat spring conference, the party's leader and deputy prime minister Nick Clegg gave his support to gay marriage, saying the "freedom to love who you choose is a fundamental right in a liberal society".
Equalities Minister Lynne Featherstone will launch a consultation later this month on how to make civil marriage available to same-sex couples. She has said she wants to challenge the view that the government does not have the right to change marriage traditions.
The Catholic journal The Tablet reports that the question of whether gay marriage should be allowed at all will be included in the consultation.
The Scottish government has held a consultation process north of the border and received more than 50,000 responses.